Wells are generally drilled into the ground to recover natural deposits of oil and gas, as well as other desirable materials, that are trapped in geological formations in the Earth's crust. A well is drilled into the ground using a drill bit at the end of a drill string. The drill bit is directed to the targeted geological location from a drilling rig at the Earth's surface. A drilling fluid, called “mud,” is pumped down through the drill string to the drill bit. The mud cools and lubricates the drill bit, and it carries the drill cuttings away from the bottom of the borehole. The mud and the drill cuttings flow to the surface through the annulus between the drill string and the borehole wall.
FIG. 1A shows a typical drilling system 100. A drill string 104 is suspended in a borehole 102 from a drilling rig 101 at the surface. A drill bit 105 at the lower end of the drill string 104 is used to drill through earth formations 103. The bottom hole assembly (“BHA”) commonly includes drilling tools, sensors, computers, and other equipment that may be used in the drilling process. For example, FIG. 1A generally shows a drill collar 107 and a drilling tool 106 that may contain these types of equipment.
FIG. 1B shows a typical wireline tool 157. The tool 157 is suspended in a borehole 152 from a rig 151 via wireline 154. Typically, wireline tools, such as tool 157 in FIG. 1B, are lowered into a well after drilling has been completed or during interruptions in the drilling process when the drill string has been removed from the well. A typical wireline tool includes sensors and other equipment that are used to investigate the earth formations and their contents.
Many of the tools, sensors, and other equipment that are used in downhole tools, such as the drilling tools and wireline tools of FIGS. 1A and 1B, require electrical power. A battery may be installed in the tool to supply electrical power to the tool when the tool is downhole.
The hydrostatic pressure of the mud in the borehole increases with increasing depth. In addition to the increased pressure, the temperature of the mud also generally increases with increasing depth. The increase in temperature is caused by the increasing heat generated in the Earth's mantle. Thus, downhole tools are often operated in a high-temperature environment. In some cases, the temperatures may be above about 125° C.
Conventional batteries are generally not well-suited for operation in a high temperature environment. In fact, most industrial and commercial battery applications include protection devices that will disable or discharge a battery if the ambient temperature becomes too high. This will prevent the battery from corroding or even exploding, in extreme cases. For example, U.S. Pat. No. 6,570,749 discloses an over-current and thermal protection device. The device will regulate the current drawn from the battery in the event that the battery temperature becomes too high.
Another device, disclosed in U.S. Pat. No. 6,025,699, is designed to close a discharge circuit when the battery temperature is high for the amount of energy stored in the battery. The discharge circuit will enable the battery to discharge to a safe level for the battery temperature.
Because the temperature in the downhole environment usually exceeds the normal operating range of a typical commercial or industrial battery, drilling tools and wireline tools are often equipped with a high-temperature battery. A high-temperature battery has an altered chemistry so that it is able to more efficiently provide electrical power at high temperature, without any danger of explosion or corrosion. For example, a high-temperature battery, designed to efficiently operate at temperatures above 125° C., is disclosed in U.S. Pat. No. 6,187,469.
A high-temperature battery, however, will generally operate inefficiently at lower temperatures. Moreover, drawing power from a high temperature battery at low temperatures may effectively consume an inordinate amount of electrical power that cannot be recovered.
What is needed, therefore, are techniques for protecting a battery of a downhole tool when it is exposed to temperatures below an operating threshold. It is desirable that an electrical circuit be provided to prevent excessive power loss from a battery when the downhole tool is exposed to temperature below a given threshold. It is further desirable, in some cases, for an electrical circuit to provide one or more of the following among others: greater reliability for battery power, flexibility in circuitry options, increased battery life, increased battery efficiency, protection for the battery, and alternate power sources.